2010 AMC 10B Problem 23

Attempt Problem 23 of the 2010 AMC 10B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AMC 10B solutions, or check the answer key.

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23.

The entries in a 3×33 \times 3 array include all the digits from 11 through 9,9, arranged so that the entries in every row and column are in increasing order. How many such arrays are there?

1818

2424

3636

4242

6060

Answer: D
Concepts:arrangements with restrictionscasework
Difficulty rating: 2030
Solution:

Let aija_{ij} be the entry in row ii and column j.j. The increasing conditions force a11=1,a_{11}=1, a33=9,a_{33}=9, and a22a_{22} to be 4,5,4,5, or 6.6.

If a22=4,a_{22}=4, then {a12,a21}={2,3}.\{a_{12},a_{21}\}=\{2,3\}. Choose which two of 5,6,7,85,6,7,8 go in the bottom-left pair {a31,a32};\{a_{31},a_{32}\}; the other two go in the top-right pair {a13,a23}.\{a_{13},a_{23}\}. Each pair then has only one increasing order. There are 2(42)=122\binom42=12 arrays: two orders for 2,32,3 around the upper-left corner and (42)\binom42 choices for the bottom-left pair. By reversing the digits and rotating the array, there are also 1212 arrays with center 6.6.

If a22=5,a_{22}=5, choose the three entries in positions a12,a13,a23.a_{12},a_{13},a_{23}. They can be any three of {2,3,4,6,7,8}\{2,3,4,6,7,8\} except {2,3,4}\{2,3,4\} or {6,7,8};\{6,7,8\}; those two choices would put all three small or all three large entries on one side and violate a required comparison with the center. Every other choice uniquely determines the remaining entries and their increasing orders. This gives (63)2=18\binom63-2=18 arrays.

The total is 12+18+12=42.12+18+12=42.

Thus, D is the correct answer.

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